RADIOLOGICAL ENGINEERING LAB MANUAL

Transcription

1 RADIOLOGICAL ENGINEERING LAB MANUAL Instructors: George Xu Peter Caracappa Chengyu (Stone) Shi SPRING 2004 Copyright Rensselaer Polytechnic Institute 2004

2 ACKNOWLEDGEMENTS This manual has been constantly updated since 1996 and many former instructors and students have contributed to this document. Much of the equipment in the Radiological Engineering Laboratory were donated to us prior to 1994 by nuclear power plants where many former instructor-professor Robert Ryan students worked. Most newer equipment were purchased with the following grants: 1. National Science Foundation. Development of An Undergraduate Laboratory Course In Environmental Health Physics, Principal investigator, Professor George Xu; NSF/ILI (#DUE ). 2. Department of Energy. Real-time Identification and Characterization of Asbestos and Concrete Materials With Radiological Contamination, Principal investigator, Professor George Xu; DOE/EM/D&D (DE-FG07-98ER62706). 3. RPI School of Engineering Laboratory Grant. 4. Department of Energy Matching Grant to NEEP Program administered through Professor Don Steiner.

3 TABLE OF CONTENTS LAB REPORT GUIDELINES..1 LAB #1. MEASUREMENT OF BETA PARTICLE RANGE.3 LAB #2. PROPORTIONAL COUNTER AND AUTOMATIC SAMPLER CHANGER SYSTEM.7 LAB #3. GAMMA SPECTROMETRY USING NAI(TL) AND HPGE DETECTORS.. 12 LAB #4. RADIATION SURVEY INSTRUMENT CALIBRATION I (BETA, GAMMA) 19 LAB #5. MOSFET DOSIMETER SYSTEM..28 LAB #6. AIR SAMPLING FOR RADON.31 LAB #7. LIQUID SCINTILLATION COUNTING.33 LAB #8. RADIATION SURVEY AND CONTAMINATION CONTROL AT RPI FACILITIES..36 LAB #9. SHIELDING DESIGN USING MONTE CARLO CODE MCNP.38

4 LAB REPORT GUIDELINES The labs for this course are meant to be instructive as to methods and procedures beneficial to the study of health physics. The procedures outlined in each experiment are as complete as possible. At times, however, it may be necessary to consult additional materials, either in the operation of the lab or the data analysis and discussion of the results. Manuals for the operation of the lab equipment will be available in the lab. Ask the lab instructor if you need assistance. Lab reports are due a week from the date that the lab is performed (the specific due date will be announced in the lab for the Navy Nuclear students). Late submissions are subject to penalty. Your lab report is not only a record of the work you have completed, but an explanation of that work to others. The report should be addressed to someone that has a basic knowledge of the principles of health physics, but may not be familiar with the particular equipment or procedures used in your lab. The report should contain the following basic elements: COVER Student Name, Team Members, Experiment Title, Date, etc. ABSTRACT A short paragraph summarizing the results obtained during the lab. A good abstract explains to the reader the purpose and importance of the lab, briefly summarizes what was done in the lab, and provides the numerical results from it. In essence, the abstract is meant to convince the reader that it is worth it to him or her read the rest of the lab. OBJECTIVE This is a well-formulated sentence (or two at the most) that explains why it is important to do this lab, and how it applies to the field of health physics. THEORY This section contains a description of the theoretical basis for the lab. It also describes the experimental setup and the experimental procedure in general terms (it is not necessary to list every detailed step of the procedure, but it should describe what actions were taken). Cite references if necessary. DATA Present data appropriately (typically in tabular and graphical forms). Please give a title and a number to figures and graphs. Be sure to include sample calculations for any data analysis or error analysis that you perform. Calculations that are incorrect without any explanation of how they came about cannot be credited. 1

5 DISCUSSION This is the most important section of the lab report. You should discuss what your data means and your confidence in the data (how good is it and why?). Also, discuss any assumptions that you made in the lab or report. You should also answer any questions that may be presented in the lab manual (not in a question-response form the questions should be addressed in a narrative way in the text). You also need to address why it is you did this lab, meaning how your objective was fulfilled. Many years from now, you may find a detailed report of yours extremely useful. GOING FURTHER What additional experimentation might you do that would contribute to using the lab as a tool of health physics? Or alternatively, how could this lab procedure be improved to better illustrate the principles it is trying to address? Obviously, reports should be typed and easy to read, and should be as professional as possible. You should write your own report and share only the data. Use as many pages as needed, but a report usually is 4-6 pages long (not including charts, figures, or attachments). Make sure you answer all the questions in the lab. Each lab will be graded out of a total of 10 points. In general, the points are broken down by: abstract (1 point), objective (1 point), theory (1 point), data (2 points), discussion (3 points), going further (1 point), and the overall appearance and quality of the work (1 point). However, this is not a strict system, and an exceptional job on one section may help boost the grade on a section that was a little weaker (but don t count on it aim for a balanced report!) ANY ADDITIONAL QUESTIONS ABOUT THE LABS OR THE LAB REPORTS MAY BE ADDRESSED TO THE LAB INSTRUCTOR. 2

6 LAB #1 MEASUREMENT OF BETA PARTICLE RANGE OBJECTIVE: Measurements will be made to determine the ranges of various beta-particle emitters and one unknown source. You will measure the transmission of beta rays through thin foil absorbers, and determine the range of the betas. Since the energy spectrum is continuous, the range should be related to the maximum kinetic energy of the betas. For accurate range measurements, you will need to make careful subtraction of "background" counting rates. REFERENCE: 1. Introduction to Health Physics by Herman Cember, 3rd Ed. (1996). 2. Nuclides and Isotopes, Fourteenth Edition, General Electric Company (1989). EQUIPMENT REQUIRED: Beta emitters (Cl-36, C-14, Tc-99, Sr-90, unknown source) Counting well Beta counter (Geiger detector) A set of foils A. INTRODUCTION During the decay of a beta emitter, beta particles (i.e., electrons) are emitted at various energies, ranging from a maximum energy to zero. The attenuation of beta particles by any absorber can be measured by interposing successively thicker absorbers between a beta source and a Geiger counter. The beta-particle counting rate decreases rapidly at first, and then slowly as the absorber is thick enough to stop all the beta particles. At this time, the counting rate stays constant due to background radiation, as shown in Figure 5.2 of the book by Cember (1996). The end point of the absorption curve is called the Range of the beta particles in the material of which the absorbers are made. Figure 5.2. Absorption curve of Bi-210 beta particles with E max being1.17 MeV. The broken line represents the mean background counting rate (Cember 1996). 3

7 The range for a beta source is dependent on the maximum beta energy of the source. The following beta sources are used to determine the counting rate vs. thickness curve. Please verify the information by checking the most recent version of Nuclides and Isotopes, General Electric Company. Cl E5 a β ε β +.12w w= absolute abundance less than 1% C a β Tc h 2.13E5 a IT β γ γ 89.7 vw β w w= absolute vw=absolute abundance less abundance less than 1% than 10E-3% Sr a β In the above isotopes, the symbol a is the half-life in years, h is in hours, and d is in days. B. DETECTOR SETUP The counting well is made of lead shielding and a stand for inserting absorbers, as shown in Figure 5.1. Figure 5.1. Experimental arrangement for absorption measurements on beta particles (Cember 1996). The ability to absorb energy from beta particles depends on the number of absorbing electrons in the path of the beta particles (electrons/cm 2 ), i.e., it is proportional to density x linear thickness (called density thickness), t d (g/cm 2 ). ρ (g/cm 3 ) * t l (cm) 4

8 Since the density for aluminum is 2.7 g/cm 3, the density thickness, t d, for 1-cm thick aluminum is 2.7 g/cm 3 x 1 cm = 2.7 g/cm 2. Information about t d are available as tables or graphs. Figure 5.4 is a typical curve for the beta maximum energy vs. the range. Figure 5.4. Range-energy curve for beta particles (Cember 1996). C. PROCEDURE (a) Connect the Geiger counter to the counting well. (b) Place a beta source on the stand. (c) Place absorber #1 between the source and the counter and then close the counting well. (d) Zero the counter reading and the counter will start counting. (e) Wait until the counter displays a stable counting result in Counts per Minutes (C/M). Record the reading and the thickness of the absorber. (f) Replace with a thicker absorber and repeat steps c) to e) until the reading is a constant. (g) Repeat above steps for all five beta sources. D. Questions and Considerations a. Enumerate the possible sources of "background" counting rates. Consider different types of radiation and their penetrating ability. Also consider if any background could be attributed to the source. What empirical method might you use to determine the background? b. Consider how best to spend your measurement time in order to obtain the most precise values of the background-corrected counting rates. 5

9 c. Beta ranges, expressed in units of mg/cm 2, are approximately equal for different absorbers. Why? d. 137 Cs undergoes beta decay to an excited state of 137 Ba, which then decays to to ground state by emitting a MeV gamma ray. Explore the possibility of determining the range of betas emitted from a 137 Cs source by th method used in this experiment. In your lab report, tabulate and plot your curves. Explain your findings in details. What is the background counting rate? Does the distance between the source and detector affect your result and why? 6

10 LAB #2 PROPORTIONAL COUNTER AND AUTOMATIC SAMPLE CHANGING SYSTEM OBJECTIVES: Measurements will be made to: (1) Determine an alpha plateau and operating voltage setting. (2) Determine a beta plateau and operating voltage setting. (3) Determine the efficiency for counting both alpha and beta samples. (4) Derive a curve of Betamax energy versus efficiency for various beta standard sources. REFERENCE: Radiation Detection and Measurement, by G.F. Knoll, 2nd Ed. (1989). EQUIPMENT REQUIRED: Beta Calibration Sources (Cl-36, Pm-147, C-14, Tc-99, Sr-90, Bi-210) Alpha Calibration Source (Pu-239) Tennelec Low Level Alpha/Beta Counting System (Model 5110/5500). Proportional Counting Gas (P-10) A. INTRODUCTION A.1. PROPORTIONAL COUNTERS A proportional counter works on the phenomenon of gas multiplication to amplify ions created by radiation interactions within a gas. Gas amplification is due to increasing the electric field to an adequate value. At low voltages, the freed electrons and ions created drift to their respective electrodes. At greater the value of the electric field, the average energy between collisions increases to the point to allow secondary ionization, which cause additional ionization. This gas multiplication becomes an ion cascade, known as a Townsend avalanche. Figure 1 illustrates the different regions of operation in gas filled detectors. In the recombination region, the detector voltage is insufficient to prevent the recombination of Pulse Amplitude (log Scale) Recombination Region Ionization Chamber Ion Saturation Proportional Counters Proportional Region Limited Prop. Region Geiger Counters Geiger- Mueller Region Applied Voltage Figure 1. Curve of pulse height versus voltage across a gas-filled pulse counter. (From G.F. Knoll: Radiation Detection and Measurements. 2nd Ed., Wiley, New York, 1989) the original ion pairs, and the collected charge is therefore less than that represented by the original pairs. In the saturation region, recombination is suppressed to a negligible level and all the original charges created in the ionization process contribute to the collected pulse. Increasing the applied voltage cannot increase the pulse 7

11 amplitude because all charges are already collected and their formation rate is constant (ion saturation). As the detector voltage increases into the proportional region, the threshold field necessary to cause gas amplification is reached. In a portion of the region, the gas multiplication is linear and the collected charge is proportional to the original number of ion pairs formed by the radiation interaction. As detector voltage increases, into the limited proportional region, some nonlinearities will be observed, caused by the pulse amplitude no longer being proportional to the original pairs. When the detector voltage enters the Geiger-Mueller region, the avalanche proceeds until a sufficient number of positive ions are created to reduce the electric field below the point that additional gas amplification can occur. In this experiment, the Tennelec LB5100/5500 Alpha/Beta gas flow proportional counter is used; it operates in the proportional region. A.2. ALPHA AND BETA PLATEAUS dn dh DPHS Count Rate Counting Curve H V Figure 2. Typical DPHS and Counting Curve for alpha particles. (From G.F. Knoll: Radiation Detection and Measurements. 2d Ed., Wiley, New York, 1989) Proportional counters can be used to distinguish between alpha and beta sources. For charged particles, such as alphas and betas, a signal is generated for each particle that deposits a sufficient amount of energy to the fill gas of the detector. Low values of gas amplification are typically used which requires that the detection of the original pulse originate from the number of ion pairs in order to exceed the discrimination level of the system. For alpha counting, the differential pulse height spectrum (DPHS) has a single isolated peak as shown in Figure 2, since nearly all pulses from the detector are due to monoenergetic alpha particles. This peak results in a simple plateau if the detector dimensions are large compared to the range of the particle, which is usually the case with alphas. For beta particles, the particle range usually greatly exceeds the chamber dimensions. The ion pairs formed in the gas are then proportional to only that small fraction of the particle energy lost in the gas before reaching the opposite wall of the chamber. Additionally, due to the non-monoenergetic nature of beta particles, the generated beta pulse will be broader than a pulse generated by alpha and will be smaller in height. This leads to the formation of a second plateau, the beta plateau, in which both alpha and beta interactions are counted. This can be seen in Figure 3. dn dh DPHS beta alpha Count Rate Counting Curve Beta plateau alpha plateau H V Figure 3. Typical DPHS and Counting Curve for alpha and beta particles. (From G.F. Knoll: Radiation Detection and Measurements. 2d Ed., Wiley, New York, 1989) Because the beta particle pulse height distribution is broader and less well separated from the lowamplitude noise, the beta plateau is generally shorter and shows a greater slope than the alpha plateau. The plateaus are used to identify operating voltages, 8

12 because the count rates are stable under these operating voltages. Once the operating voltages are determined for a counting system, they should be preset for the system. To count an unknown sample, the counter will be automatically operated at the preset voltages for alpha and beta, respectively. A.3. ALPHA AND BETA SOURCES In this lab, the Pu-239 is used as a standard alpha source and Cl-36 is used as the standard beta source. Each of the sources is marked with its activity (usually in µci) and the date the activity was determined. To ensure the activity is correct for present day use, the radioactive decay must be taken into account. This is accomplished by the following expression: λ At () = A( 0) e t where A(t) is the activity at some time t, A(0) is the initial activity at t = 0, λ is the decay constant =.6931/t 1/2, t 1/2 is the half life, and t is the time desired for known activity from t = 0. A.4. COUNTING EFFICIENCY To determine counting efficiency the following expression is used: Efficiency = [(cpm - background)/dpm] x 100% where the cpm are obtained from the detector output and the dpm are determined from the source. The following sources were used in the plateau determination (obtained from Nuclides Pu E4 a α 5.156, 5.143, 5.105,.. Cl E5 a β ε β +.12 w w= absolut abundance than and Isotopes, Fourteenth Edition, General Electric Company, 1989): Pu-239 is primarily an alpha emitter of various energies. Cl-36 is primarily a beta emitter, however it undergoes electron capture and does produce beta plus particles at a low abundance. The beta counting efficiency is dependent on the maximum beta energy of the source. The following beta sources were used to determine the counting efficiency versus beta max curve. Please verify the information by checking the most recent version of Nuclides and Isotopes, Lockheed Martin Company. 9

13 Pm a β γ C a β Tc h 2.13E5 a IT β γ γ 89.7 vw β w w= absolute vw=absolute abundance less abundance less than 1% than 10E-3% Sr a β Bi E6 a 5.01 d α β α vw γ γ 305 vw 266 w= absolute abundance less than 1% vw=absolute abundance less than 10E-3% In the above isotopes, the symbol a is the half life in years, h is in hours, and d is in days. C. PROCEDURE The Tennelec computer control system is fully automated, and the settings on the device are not typically changed. The teaching assistant will make sure that the system is correctly set up before the lab begins. If the system does not appear to be functioning properly, please see the TA. C.1. PLATEAU GENERATION (a) Load only the Pu-239 (alpha) and Cl-36 (beta) standards. Set up planchets in automatic sample changer in the following order: 1. Metal Plate 2. Blank Planchet 3. Group Planchet 4. Sample 1 (Alpha, Pu-239) 5. Another Group Planchet 6. Sample 2 (Beta, Cl-36) 7. Metal Plate at the end. (c) Click on the GO button on the icon bar at the top of the screen. From the list of items, choose LAB Plateau Alpha and select Open. On the left of the dialog box, select the letter of the group plate that precedes the alpha source, and click OK. The system will begin to move. Then click the GO button again and select LAB Plateau Beta and Open. Select the letter of the group plate preceding the beta source and click OK. A second page of data collection will open, but nothing will be collected until the alpha counting is complete. You can switch back and forth between the pages using the Window pull-down menu. The counting procedure is designed to step from 495 volts to 1515 volts at intervals of 30 volts. Each count takes one minute, so the system will count for a little over an hour with no intervention. The report should print automatically when the procedure is complete. If it does not, see the TA. 10

14 (c) Analyze the plot and verify that you agree with the operating voltage that the computer has chosen. C.2. COUNTING EFFICIENCY (a) Load planchets in the system with a metal plate first, a blank planchet, a group planchet, a planchet with the Pu-239 source, and planchets with the beta sources that you have available to you (be sure you know which order they are in). (b) Run the LAB Efficiency procedure. (c) The system will count each source for two minutes, and print out the average counts per minute (cpm). Background is already corrected for in this value. (d) The disintegration per minute (dpm) for the standards are known. Calculate counting efficiency of the system for alphas and betas by: Efficiency = [cpm/dpm] X 100%. (e) Use the standard deviation reported by the computer to calculate the standard deviation in the efficiencies for each source. (f) For the efficiencies calculated for each beta source, plot a curve of efficiency vs. Beta max energy. Information on Beta max energy can be found on the Chart of Nuclides in the lab manual. (Please remember that Sr-90 decays to yield Y-90 which emits beta particles at a maximum energy of 2.28 MeV.) C. 3. CROSSTALK MEASUREMENT In order to differentiate between alpha and beta counts, a specific energy level is chosen to discriminate between the two. If counts from one type of source are counted in the other region, this is called crosstalk. Estimate the crosstalk in the system by setting it up to count a single planchet that is holding both the Clorine-36 and the Plutonium-239 sources (be careful not to damage the Pu-239 source, or to lay the sources on top of each other). Run the efficiency procedure, and compare the alpha and beta counts in this measurement to the counts generated for those sources in the last section of the lab. D. QUESTIONS Consider these additional questions in your discussion: (a) Why is advantageous to operate in the plateau region? (b) The system corrects for background using measurements previously taken over very long counting times. What is the source of that background, and why is it so much higher for beta than it is for alpha? (c) Describe a possible use for this device in radiation protection or health physics, and explain how these experimental procedures would be used in it. 11

15 LAB #3 GAMMA-RAY SPECTROSCOPY USING NaI(Tl) AND HPGE DETECTORS OBJECTIVES: (1) To gain familiarity with the NaI(Tl) and HPGe systems through use of standard sources. (2) To compare gamma spectroscopy with the NaI (Tl) and HPGe detectors. (3)To determine the composition of an unknown from its gamma energy spectrum (isotope identification). REFERENCES: Radiation Detection and Measurement by G. F. Knoll, 2 nd Ed. (1989) The Gamma Rays of the Radionuclides by G. Erdtmann and W. Soyka (1979) Scintillation Spectrometry - Gamma Ray Spectrum Catalog by R. L. Heath EQUIPMENT REQUIRED: NaI(Tl) Scintillation Detector, Photomultiplier Tube and Base, and Pb Shield HPGe Portable Detector IBM-PC Computer and Genie-2000 Analyzer InSpector2000 Multi Channel Analyzer and High Voltage Supply Standard Gamma Sources and Unknown Source A. INTRODUCTION A.1 GAMMA-RAYS The gamma-ray energies emitted by various radionuclides are unique for each species; and they are said to fingerprint the material. Determination of the quality and quantity of a gamma spectrum can identify minute quantities of an element within an unknown sample. In this laboratory, you will acquaint yourself with the Thallium-activated Sodium Iodide (NaI(Tl)) and High Purity Germanium (HPGe) systems through use of standard sources. You will set up, energy calibrate, and determine the efficiency and resolution of the gamma spectroscopy systems. Ultimately, you will identify/determine the composition of an unknown sample. X-ray or gamma-ray photons are uncharged particles. They create no direct ionization or excitation of any material through which they pass. The detection of gamma-rays is critically dependent on causing a gamma-ray photon to undergo an interaction that transfers all or part of the photon s energy to an electron in the absorbing material. Because the primary gamma-ray photons are invisible to the detector, it is the fast electrons created in the gamma-ray interactions that provide information of the incident gamma-rays. 12

16 Z of absorber Photoelectric Effect Dominant Compton Effect Dominant Photon Energy (MeV) Pair Production Dominant log scale Figure 1. Relative importance of the three major types of gamma-ray interactions. (From Radiation Detection and Measurement, 3rd Ed, by G.F. Knoll, John Wiley & Sons, Inc,1989) The interaction mechanisms of gammarays include photoelectric absorption (predominant at low-energy gamma-rays and high Z material), pair production (predominant at high-energy gamma-rays and high Z material), and Compton scattering (most probable process over the range between the two extremes). These interactions can be characterized as shown in Figure 1. For a detector to serve as a gamma-ray spectrometer, it must act as a conversion medium in which incident gamma-rays have a reasonable probability of interacting to yield one or more electrons and it must function as a conventional detector for these secondary electrons. A.1. SCINTILLATION DETECTOR MECHANISM NaI(Tl) is an alkali halide inorganic scintillator. As you will recall from lab 4, a scintillator is a detector that converts the kinetic energy of an ionizing particle to a flash Band Gap Conduction Band Scintillation Photon Valence Band Activator Excited States Activator Ground State Figure 2. Energy band structure of an activated crystalline scintillator. (From Radiation Detection and Measurement, 3rd Ed, by G.F. Knoll, John Wiley & Sons, Inc,1989) of light. The scintillator mechanism depends on the energy states determined by the crystal lattice. The inorganic scintillator is based on the energy band structure shown in Figure 2. Incident gamma-ray energy is absorbed and produces secondary electrons. The charged particles passing through the medium or deposited photon energy will form a large number of electron-hole pairs created by the elevation of electrons from the valence band across the forbidden band gap to the conduction band. The return of the electron to the valence with the emission of a photon is an inefficient process. It can be enhanced by the presence of an activator. The activator, or an impurity added to the inorganic material, creates energy bands in the band gap through which the excited electron can deexcite back to the valence band. Because the energy is less than that of the band gap, this transition can now give rise to a visible photon and therefore serve as the basis of the scintillation process. One important consequence of luminescence through activator sites is the fact that the crystal can be transparent to the scintillation light. The scintillation photons are converted to electrons and multiplied in the photomultiplier tube. Since the activator site is formed in an excited configuration with an allowable transition Scintillator & Photomultiplier HV (to PM Tube) Preamp Amp MCA Figure 3. Electronic Setup of experiment to the ground state, its deexcitation will occur very quickly and with high probability for the emission of a corresponding photon. 13

17 Because the migration time of the electron is much shorter, the excited impurity configurations is formed essentially at once and will subsequently deexcite with the halflife characteristic of the excited state. It takes normally three times the band gap energy to create one electron-hole pair. NaI(Tl) uses Thallium Iodide as the activator (0.1% by mass). These scintillators are hygroscopic (deteriorate due to water absorption if exposed to atmosphere) and therefore need to be placed in an air-tight container. These detectors have excellent light yield and linearity. The electronic setup for the detector is shown in Figure 3. A.2. SEMICONDUCTOR DETECTOR MECHANISM The HPGe Detector is a semiconductor diode type detector. The detector is formed by setting up two different regions in the semiconductor, p ( excess hole concentration) and n (excess electron concentration), which have a region between them where a charge imbalance exists. This region is the depletion region, and it extends into both the p and n sides of the junction. A p and n region adjacent to one another is called a junction. P connected to the positive and n connected to the negative is the forward bias. The impedance across the junction is low and the current will flow across.) If the number of charges on both sides are equal, the region is the same on both sides. Electron-hole pairs formed in the depletion region by the electric field and their motion constitutes the basic electrical signal. Thus, the semiconductors provide a way to collect electrical charges created at either boundary of the semiconductor material. By reverse biasing the p-n junction, very low current will cross the junction. The width of the depletion region increases and the noise properties of the semiconductors improve. When the reverse bias voltage is sufficient, the depletion region extends through an entire thickness of the wafer, resulting in a fully depleted detector. The depth of the depletion region is important to the types of radiation that can be detected. In the case of gamma spectroscopy, the deletion depth should be as large as possible. The thickness of the depletion region, d is given by V d = 2ε en where ε is the dielectric constant of the medium, V is the reverse bias voltage and N is the net impurity concentration in the bulk semiconductor material. Gamma radiation requires greater depletion depths. Greater depletion depths are obtained by lowering N through further reductions in the net impurity concentration. Germanium (Ge) is of high purity, however it is further processed to higher purity by the process of zone refining. The impurity levels are progressively reduced by locally heating the material and slowly passing a melted zone from one end of the sample to the other. Since impurities tend to be more soluble in the molten germanium than in the solid, impurities are preferentially transferred to the molten zone and are swept away from the sample. Because of the small band gap ( 0.7 ev), room-temperature operation of germanium detectors of any type is impossible because of the large thermally induced leakage current that would result. HPGe must be cooled through the use of liquid nitrogen (typically) to reduce the leakage current. The detector must be placed in a 12 / 14

18 vacuum-tight cryostat to inhibit thermal conductivity between the crystal and the surrounding air. High resolution is typical of the HPGe due to the very low energy required to produce electron-hole pairs so that a large number of charge carriers are produced, resulting in good statistics. The electronic setup for the detector is shown in Figure 4. HPGe Detector w/cryostat PreAmp Amp MCA Dewar w/ Liquid Nitrogen HV Figure 4. Electronic Setup of experiment The preamplifier is normally incorporated as part of the cryostat package in order to minimize capacitance. B. PROCEDURE B.1. SODIUM IODIDE SCINTILLATION DETECTOR Most of the connectors for the system will be in place before the lab begins, but there are two that will have to be put in place before you operate the detector. They will both be labeled. One will be for the high voltage (labeled HV), and one for the signal (labeled Signal or Energy). These are so that you can switch between the two different detectors used in the lab. *CAUTION: The system is set up by the laboratory instructor. The high voltage level is set in the program setup, and should not be adjusted by the student. Do not shut off high voltage or adjust HV, or disconnect the HV while the program is running. Damage to the detector can result. Record voltages and amplifier settings. 1. Load the file for the sodium iodide detector from the list of detector setups. Do not adjust the settings for this configuration. You may need to verify that the HV is turned ON. This is done through choosing MCA on the menu bar, and choosing Adjust. This will pop up an additional window, with several pages (indicated by the menu bar at the top). Under PwrMgr, the Power Mode should be listed as AC Full, and under HVPS, the power should be listed as On. It takes one minute before the sign "Wait" to disappear from the menu. 2. The system must first be calibrated for energy. Choose several sources from the calibration set and place them in the detector at the same time. Allow the InSpector to collect data until the peaks are well defined and discernible. This may be less than the preset time of 600 seconds. The program will provide rough estimates of the peak 15

19 energies. That estimate and relative positions should allow you to determine which peak corresponds to which gamma ray energy. In the Calibration menu, choose "Calibration by Energy only." Choose the energy with the cursor by placing the cursor at the top of the peak, and input the energy. Do this for all the energies that you can determine, and run the calibration. Use "Show" to check for linearity of the calibration curve before accepting the energy calibration. Use "Save As" under the "File" to save the spectrum to the computer. 3. Plot the gamma energy vs. channel # for all peaks used for energy calibration. 4. Remove all sources from the detector and clear the spectrum from the screen (be sure you have saved it already). The next step is to measure the background from the detector. Allow the detector to collect data for at least twice as long as was taken in step two, and not less than two minutes (120 seconds). Note the total live time, and save this spectrum to another file. You will use it later. 5. Place the Cs-137 source back in the detector and collect a spectrum as in step two. 6. Under the Options menu, choose Strip. This will strip the background from the spectroscopy results. Choose the file that you saved te background spectrum to. At the top of the window, there will be a place to enter a multiplication factor for the file. Correct the background counts so that the background spectrum is normalized to the time that the current spectrum has been collected. After stripping, the spectrum contains only sample peak information. 7. Determination of Peak Area. The Peak Area is the total counts under the full-energy gamma-ray peak (or the photopeak) and is established by defining the Region of Interest (ROI ). The left and right bounds of the ROI can be moved by dragging them from the sides of the display, or by use of the CTRL-L and CTRL-R keys. Information of the ROI is summaried at the bottom of the screen. After the background stripping, the photopeaks are often still superimposed on a continuum that is caused by interactions of the gamma-rays with the detector. The counts shown as "Area" is the "peak area" without the continuum, and the counts shown as "Integral" include the continuum under the peak. 8. The system is now ready to be calibrated for counting efficiency. This is done by dividing the peak area (without the continuum) with the activity of that sample, i.e., Efficiency (E) % = (Peak Area in Counts per Second)/(Sample Activity). Perform efficiency calibration for all photopeaks. The original activities of the standards and half-lives are used to determine the Sample Activity. Plot the counting efficiency (%) vs. gamma-ray energy in MeV. NaI detectors have better counting efficiencies (~ 5-10%) than the HPGe detectors (~0.1-1%). 16

20 7. Energy Resolution of a system is determined by the width of a photopeak. Using the Cs-137 photopeak, the energy resolution is given by: Energy Resolution (%) =(FWHM in kev)/(peak Energy in kev), Where FWHM = Full width of peak at half of the maximum peak height. NaI detectors have worth energy resolutions than HPGe detectors. 8. Repeat step 6 only using the other standard sources provided to you, for all the photopeaks that you can identify (you may choose to put more than one standard in the detector at a time). 9. Identify the unknown sample provided by instructor. B.2. HIGH PURITY GERMANIUM (HPGE)DETECTORS CAUTION: The system is set up by the laboratory instructor. The high voltage level is set in the program setup, and should not be adjusted by the student. Do not shut off high voltage or adjust HV, or disconnect the HV while the program is running. Damage to the detector can result. NOTE: When placing the samples in front of the HPGe detector window, it is desirable to use a reproducible geometry (i.e., place each source in the same location) in order to generate meaningful results. 1. Close the detector file that was active. Disconnect the High Voltage and Signal connectors from the Sodium Iodide detector and replace them from the ones from the High Purity Germanium Detector. Load the detector file for HPGe detector. 2. Perform energy calibrate for the system using the same procedures as described above. 3. Determine the energy at the peak channels on the calibrated system. Graph energy vs. channel no. for these points. 4. Measure background for the instrument and perform counting efficiency calibration for the system. Plot counting efficiency (%) vs. gamma energy (MeV). 6. Determine the energy resolution using Cs-137. Compare with result for NaI detector. 7. Verify characteristic peaks for various isotopes (Co-60, Cs-137 and Ba-133 standards). 8. Identify unknown provided by the lab instructor and repeat the above process. 17

21 REPORT Printouts of the data collection may be made from the computer. It might be useful to have printouts of the different detector operations for use in your data analysis and your discussion. Make sure to answer all the questions in the narrative. Gamma-rays are uncharged and thus create no direct ionization or excitation of the material through which it traverses. The detection of gamma rays is critically dependent on interactions to cause the gamma-ray photon to transfer energy to an electron in the absorbing material. Gamma rays are also very penetrating. The Thallium-activated Sodium Iodide -NaI(Tl) is based on scintillation principles whereas the High Purity Germanium detector is based upon semiconductor principles. Discuss the detector principles and detector set-up (from when a photon enters the detector to a signal on the computer-channel accumulation). Discuss energy resolution (the separation of closely spaced gamma-rays), channels number vs. distinction between various peaks, the advantages disadvantages of the two detection (photopeak efficiencies, etc.). Compare and contrast the use of these two different instruments in the field of health physics. When and why would you choose one over the other? 18

22 LAB #4 RADIATION SURVEY METER CALIBRATION OBJECTIVES: To calibrate portable survey meters for alpha, beta, and gamma radiation. REFERENCE: Radiation Detection and Measurement, by G.F. Knoll, 2nd Ed. (1989). EQUIPMENT REQUIRED: Cs-137 Source Alpha Source & Calibration Beta Sources & Calibration Meter Stick Ludlum Survey Meter Ludlum 43-2 Alpha Probe Ludlum Beta Probe Portable Ion Chambers and GM counters for gamma radiation A. INTRODUCTION In this experiment, radiation survey meters for alpha, beta, and gamma sources will be calibrated. In this experiment, the Cs-137 gamma source and alpha and beta check sources will be used as standard sources (i.e., they have been pre-calibrated against NIST traceable sources). The calibrations are necessary and should be repeated every year, following repair, or when deemed necessary, i.e. thought to be out of calibration. Calibration should be considered acceptable when the instrument responds to better than ±10% of the radiation level being measured either by a direct meter response or a predetermined calibration curve. For alpha and beta measurements, we are typically interested in the activity of the sample (i.e. the number of counts measured). For gamma sources, survey meters may measure counts, but are more likely to be used to estimate dose. B. SOURCES Several different sources will be used in this lab to calibrate the survey meters. Y d 90 Sr/ 90 Y are β- emitters. The 90 Sr emits as follows: β Sr a β Sr Y + β + ν The beta particle has a continuous energy spectrum with β max = MeV. The decay has a long half-life of 29.1 years. 19

23 The 90 Y is also a β emitter with a continuous energy spectrum. The β max = MeV with a short half life of 2.67 days. Since the parent has a long half life and the daughter has a relatively short half life, these decays are in secular equilibrium. Pu E4 a α 5.156, 5.143, 5.105,.. Another source used in the experiment is 239 Pu. This radioactive particle emits alpha particles of several energies (5.156 MeV, MeV, and MeV) with a very long half life of 2.41 x 10 4 years. The Pu-239 decay scheme is as follows: 239 α 235 α 231 Pu U Th... where the half life of the 235 U is 2.04 x 10 8 years. The counts obtained from the 239 Pu source will be those of 239 Pu primarily. When calibrating with the gamma source, there are a couple other details to consider. The survey meters can detect a large range of radiation, depending on which scale they Gamma Source t Pb Shield (Attenuator) x good geometry Detector bad geometry Figure 1: Pb Shield attenuates gamma radiation reaching the detector. An exponential transmission curve can be measured under good geometry conditions. are set on. When performing a calibration, attempts should be made to check all the different scales on each meter. To accomplish this, attenuators must be used to dampen the gamma exposure. Here, we will use ½ thick lead shields. This experiment assumes that the Pb shields are of good geometry. This means that the gamma radiation from the source is collimated and narrow enough to ensure that gamma rays from the source which escape interaction with the shield can be counted by the detector. This isn t always the case. The detector can also detect radiation that was scattered by the shielding or other types of secondary photon radiation. Under these conditions, the detector signal will be larger than assumed with good geometry. Therefore, broad beam or bad geometry, requires the use of a build-up factor, to account for gamma radiation from secondary effects in the attenuator (shield). Broad beam or bad geometry can be the result of a thick shield or a broad, non-collimated beam. Figure 1 depicts this arrangement. C. RADIATION DETECTION C.1. DETECTOR PRINCIPLES The emission of visible light when radiation is incident upon a certain material is known as scintillation. The most widely used scintillators include inorganic alkali halide crystals (such as NaI) and organic liquids. Inorganic scintillators have higher efficiency of converting the 20

24 radiation s energy into detectable light and their response is more linear the generated light is proportional to the deposited energy over a wide range. Organic scintillators have shorter decay times of the induced luminescence so a fast signal pulse can be generated. In this experiment, both detectors used are inorganic. The scintillation mechanism in inorganic materials depends on the energy states determined by the crystal lattice of the material. The left side of figure 1, below, shows that electrons have only discrete energy bands available in materials classified as insulators or semiconductors. The lower band, called the valence band, represents those electrons that are bound at lattice sites and the conduction band represents those electrons that are sufficiently energetic to migrate freely within the crystal. In a pure crystal, the region between the valence and conduction bands is called the forbidden band. A large amount of energy is required to excite an electron sufficiently to move from the valence to the conduction band (leaving a hole in the valence band), and the emission of a photon when a conduction band electron fills a valence hole is inefficient process. To enhance the probability of a visible photon being generated during the de-excitation process, small amounts of an impurity are added to a pure crystal. These impurities, called dopants or activators, create special sites in the crystal lattice with a modified energy band structure, and are shown in the right side of figure 1. In these regions, energy states are created in the forbidden band, which now constitutes the band gap, and serve as the basis for the scintillation process. Figure 2: Scintillation Material Pure Crystal Activated Crystal Conduction Band Conduction Band Activator excited states Forbidden Band Band Gap Activator ground state Valence Band Valence Band A charged particle passing through the scintillation material will form large numbers of hole-electron pairs. The positive hole will quickly migrate to the location of an activator and ionize it, because the ionization energy of the impurity will be less than that of a typical lattice site. The freed electron will move through the crystal until it encounters an ionized activator. When it does, the electron can fall into an allowed energy level in the activator site, creating a neutral impurity configuration. De-excitation of this electron to the activator ground state will occur quickly, with the emission of a visible photon if the dopant has been chosen correctly. The crystal is essentially transparent to this emitted light. This is because the energy of the emitted photon from the activated crystal is less than full energy of the forbidden band, and thus will not be influenced by the optical absorption band of the bulk of the crystal. 21

25 This experiment uses scintillators made of bismuth germanate {BGO} and silveractivated zinc sulfide {ZnS(Ag)}. Bismuth germanate (Bi 4 Ge 3 O 12 ) has a large density (7.3 g/cm 3 ) and a high atomic number which result in BGO having the largest probability per unit volume than any commercial scintillator for the photoelectric absorption of gamma radiation. BGO detectors are more rugged than those using fragile and hygroscopic NaI(Tl) crystals. BGO is a pure inorganic scintillator, since it does not rely on an activator to promote scintillation. The optical transition of the Bi +3 ion generates the luminescence, but little self-absorption of this light occurs due to special properties of the BGO crystal. The zinc sulfide scintillator has a very high scintillation efficiency comparable to that of NaI(Tl), but is only available as a powder. It is limited to thin screens used for alpha detection. Thickness greater than 25 mg/cm 2 are unusable because of the opacity of the multicrystalline layer to its own luminescence. These scintillation screens played a key role in the early experiments of Ernest Rutherford, in which alpha particle interactions were visually observed through a low power microscope. For alpha and beta measurements, the efficiencies of the probes will be calculated from the standard sources provided. The detector efficiency is given by cpm with source - cpm background dpm source 100% The value for the dpm of the source must take into account the decay of the material. The initial activity and date will be provided with the sources. The beta detector will commonly pick up other types of radiation, including low energy gamma rays. To determine the proportion of the radiation detected that is actually beta radiation, the beta factor is used. To calculate the beta factor from a standard source, the source is measured with the detection window unobstructed (ow) and again with the window covered (cw). The beta factor is defined to be: Beta factor = cpm open window - cpm closed window cpm open window In the measurement of beta detection efficiency, the net beta counts would therefore be equal to the gross open window counts multiplied by the beta factor. That number need not be further corrected for the background counts, that is included in the closed window measurement. This may seem like a roundabout way of getting the net beta counts, but the beta factor is of additional use in estimating contamination levels in a radiation survey. The other concept important in efficiency measurement is the concept of solid angle. When a material undergoes radioactive decay, it gives off radiations in all directions with an equal probability. However, a detector will only be able to register the fraction of the radiations that are directed at it. When efficiency for a detector is quoted, it typically is 22

26 given assuming that the detector was actually operating in a 4π or a 2π geometry, though that is rarely the case. Considering the radiation source as a point source, the solid angle, Ω, is defined as the number of steradians in the cone that intersects the detector. Source Ω Detector A simpler way to think of solid angle is to consider a sphere with a unit radius. The surface area of such a sphere would be 4π. If only one half of the sphere is considered, the surface area would be 2π. The solid angle of the hemisphere is also 2π. In general terms relating to a detector, the ratio of the area of the area of the detector to the area of a sphere with a radius the same as distance between the source and the detector. C.2. DETECTORS The following detectors are used in this experiment. Model Model 43-2 Ludlum Model BGO Scintillator This is a beta radiation and low energy gamma detector, consisting of a 1" (2.5 cm) diameter by 1mm thick BGO scintillator. It has a 11.6 cm 2 aluminized mylar window (ρ = 1.2 mg/cm 2 ) and a 5" (3.8cm) diameter magnetically shielded photomultiplier. Ludlum Model 43-2 Alpha Scintillator 23

27 This is a alpha survey detector consisting of a ZnS(Ag) scintillator with a 12 cm 2 aluminized mylar window (ρ = 0.8 mg/cm 2 ). Model Ludlum Model Data Logger This is a scaler / ratemeter for use with many different detectors. As a scaler, the count time can be set from 1-65,535 seconds in 1 second intervals. As a digital ratemeter, it is corrected for dead time and calibration constant. It has a 5 decade logarithmic bar graph to indicate trends and a 6 digit digital display. It can display in scales of Rem/hr, Sv/hr, R/hr, CPM, CPS, DPM, DPS, Rad (r), Gray (G), C/kg, Ci/cm 2, or Bq/cm 2. In this experiment, both detectors were reported in CPM. Depending on the availability on the day this experiment is conducted, different survey instruments will be used. The following is a description of some of the common meters: Ludlum Model 9 Ludlum Model 9 This is an ion chamber (shown above) that reads in units of mr/hr on a scale which ranges from 0-5. A switch selects between multipliers of x1, x10, x100, and x1000 of the scale reading, leading to detection ranges of 0-5, 0-50, 0-500, and mr/hr. It has a speaker that enhances surveying by providing an audible indication of exposure. D. PROCEDURE 24

28 D.1. ALPHA DETECTION Alpha survey meters are calibrated by placing the alpha detector directly on top of the alpha source. The response of the survey meter should then be noted and compared with the source calibrator (source output). 1. Attach the 43-2 Alpha probe and make sure that the base unit is set to read from Detector 2 (enter D2 on the keypad). 2. The first measurement should be without any source in place, to obtain a background reading. If the reading fluctuates, estimate an approximate background level. 3. Take the 239 Pu standard source and measure the activity with the probe. Calculate the probe efficiency Measure the detector efficiency with one or more of the other standard alpha sources. Results 1. Record readings of standard sources. 2. The given detector efficiency is reported at 60% for Pu-239 at 2 π geometry. How does the calculated efficiency compare to this? What is a possible explanation for any variance? 3. Record the efficiencies using other standard sources. How do these compare to the efficiency from the 239 Pu source? B.2. BETA DETECTION Beta survey meters are calibrated by calculating both the beta factor and the beta detection efficiency. 1. Attach the Beta probe, and set the unit to read from Detector 4 2. Begin with the C-14 standard beta source, and take measurements with the detector cover in place and removed. Calculate the beta factor. 3. Repeat the procedure with several other standard beta sources available. Results 1. Record OW and CW readings. 2. Calculate beta factor. 3. Calculate beta detection efficiency. 4. The beta detection efficiency is reported at 20% for C-14 at 2 π geometry How does this compare to the calculated efficiency? What explanation is there for any discrepancy? 5. Does the beta factor and detector efficiency vary for other sources? Why? 25

29 D.3. GAMMA DETECTION 1. le 2The Cs-137 source output has been previously determined and is available in the lab. Using the information for exposure dose rate vs. distance, determine a combination of various attenuators and distance to provide three exposure dose rates for each range of the survey meter. (The 20%, 50% and 80% Full Scale Readings). Follow the manufacturer's instructions for operation of the survey meters. Check the survey meter to assure it is working properly (battery check, zero, etc.). 2. Place the survey meter at the pre-determined distance with the necessary attenuators in place. 3.Turn the survey meter on to the proper range. 4. Assure everyone is behind the shield. 5. Turn the source on by pulling the source handle to refusal (alarm light be activated). 6. Record instrument reading. 7. Repeat for various survey meter ranges at predetermined distances and other attenuators. 8. Return source to stored position when not in use. Results 1. Explain briefly how the survey meter will be used in health physics operation. 2. Plot the exposed dose rate vs. the meter response. 3. Does the survey meter you calibrated meet the criteria of 10% at each of the 3 points on each scale? A.2. EFFICIENCIES The efficiencies of the probes will be calculated from the standard sources provided. The detector efficiency is given by cpm with source - cpm background dpm source 100% The value for the dpm of the source must take into account the decay of the material. The initial activity and date will be provided with the sources. The beta detector will commonly pick up other types of radiation, including low energy gamma rays. To determine the proportion of the radiation detected that is actually beta radiation, the beta factor is used. To calculate the beta factor from a standard source, the source is measured with the detection window unobstructed (ow) and again with the window covered (cw). The beta factor is defined to be: 26

30 Beta factor = cpm open window - cpm closed window cpm open window In the measurement of beta detection efficiency, the net beta counts would therefore be equal to the gross open window counts multiplied by the beta factor. That number need not be further corrected for the background counts, that is included in the closed window measurement. This may seem like a roundabout way of getting the net beta counts, but the beta factor is of additional use in estimating contamination levels in a radiation survey. 27

31 LAB #5 MOSFET DOSIMETER SYSTEM OBJECTIVES: Measurement will be carried out to determine the calibration factor of MOSFET dosimeters, and to study the linearity of MOSFET dosimeters. REFERENCE: 1. Introduction to Health Physics, Cember H., Introduction to Radiological Physics and Radiation Dosimetry, Attix, F.H., Operator s manual for the MOSFET AutoSense TM system, Thomson & Nielsen Ltd., EQUIPMENT REQUIRED: 1. AutoSense TM system including four MOSFET dosimeters, bias supply, and readout system 2. Cs-137 source 3. Ionization chambers 4. Portable survey meter INTRODUCTION: The basic Metal Oxide Field Effect Transistor (MOSFET) structure is depicted in Fig.1. The type shown is a P channel enhancement MOSFET which is built on a negatively doped (n-type) silicon. When a sufficiently large negative voltage is applied to the polysilicon gate a significant number of minority carriers (holes) will be attracted to the oxide/silicon surface from both the bulk silicon substrate and the source and drain regions. Once a sufficient concentration of holes have accumulated there, a conduction channel is formed, allowing current to flow between the source and drain (Ids.) The voltage necessary to initiate current flow is known as the device threshold voltage (VTH). 28

32 When a MOSFET device is irradiated, electron-hole pairs are generated within the silicon dioxide by the incident radiation. Electrons, whose mobility in SiO2 at room temperature is about 4 orders of magnitude greater than holes, quickly move out of the gate electrode while holes move in a stochastic fashion towards the Si/SiO2 interface where they become trapped in long term sites, causing a negative threshold voltage shift (DVTH ) which can persist for years. The difference in voltage shift before and after exposure can be measured, and is proportional to dose. MOSFET dosimeters are now used as clinical dosimeters for patients who undergo radiotherapy. The main advantages of MOSFET devices are: 1. The dosimeter is direct reading with a very thin active area (less than 2 mm). 2. The physical size of the MOSFET when packaged is less than 4 mm The post radiation signal is permanently stored and is dose rate independent The MOSFET AutoSense TM (TN-RD-60) is ideal for on-line advanced radiotherapy applications. Fig. 2 is a typical setup of the AutoSense TM system. This system is compatible with all Thomson Nielsen isotropic MOSFET dosimeters. Figure 2. Operational paradigm PROCEDURES A. Calibration factors for MOSFET dosimeters 1. Select the ionization chamber that has the right range for this measurement. 2. Use the reader for ionization chamber to discharge it to zero. 3. Set up MOSFET dosimeters and ionization chambers at around 25-cm-away from the Cs-137 source. 4. Click Start in the AutoSense TM PCSoftware. 5. Pull out the Cs-137 source. 6. Set timer for 30 min and start it. 29